1.  Rollout^{1}  Bar/22  eq: 0.377 
 Player: Opponent:  39.08% (G:16.68% B:1.30%) 60.92% (G:17.93% B:0.98%)  Conf.: ± 0.008 (0.384...0.369)  [100.0%] Duration: 32 minutes 36 seconds 

2.  Rollout^{1}  Bar/24 8/6  eq: 0.479 (0.103) 
 Player: Opponent:  35.34% (G:15.83% B:1.00%) 64.66% (G:16.76% B:0.79%)  Conf.: ± 0.008 (0.488...0.471)  [0.0%] Duration: 34 minutes 36 seconds 

3.  Rollout^{1}  Bar/24 7/5  eq: 0.577 (0.200) 
 Player: Opponent:  33.00% (G:14.87% B:0.99%) 67.00% (G:20.24% B:1.34%)  Conf.: ± 0.012 (0.589...0.565)  [0.0%] Duration: 32 minutes 12 seconds 

4.  Rollout^{1}  Bar/23 9/8  eq: 0.584 (0.207) 
 Player: Opponent:  33.00% (G:13.81% B:1.05%) 67.00% (G:17.44% B:0.72%)  Conf.: ± 0.008 (0.592...0.576)  [0.0%] Duration: 30 minutes 17 seconds 

5.  Rollout^{1}  Bar/23 3/2*  eq: 0.672 (0.296) 
 Player: Opponent:  31.95% (G:15.53% B:2.54%) 68.05% (G:28.03% B:2.26%)  Conf.: ± 0.010 (0.682...0.662)  [0.0%] Duration: 33 minutes 27 seconds 

6.  Rollout^{1}  Bar/24 9/7  eq: 0.723 (0.346) 
 Player: Opponent:  30.84% (G:12.57% B:0.86%) 69.16% (G:20.21% B:1.09%)  Conf.: ± 0.011 (0.734...0.712)  [0.0%] Duration: 30 minutes 01 second 

7.  Rollout^{1}  Bar/24 4/2*  eq: 0.744 (0.367) 
 Player: Opponent:  32.19% (G:16.16% B:2.62%) 67.81% (G:30.05% B:3.87%)  Conf.: ± 0.010 (0.754...0.734)  [0.0%] Duration: 33 minutes 39 seconds 


^{1} 5184 Games rolled with Variance Reduction. Dice Seed: 85576101 Moves: 3ply, cube decisions: XG Roller

In this post, two checkerplay problems were posed:
(a) How should White play 21?
All three respondents correctly supported U (Up, bar/22) as White's best move. Indeed, it is a "wipeout" (opposite of a whopper, by the B/W error scale), as the rollout above shows.
Making the sixpoint prime is not even the second best play. It is fourth best and a doublewhopper! Keeping the back men split, thereby creating more numbers to advance the back checker(s) and/or escape the prime in upcoming turns, does better.
After White correctly plays U, Blue stands better but is nowhere near a double. [White can also deflect a double (barely) if she makes the prime; that play is nonvolatile though lame by comparison.]
(b) How should White play 42?
For this second checkerplay problem (from the same board position), only kruidenbuiltje backed the correct answer. If White plays passively, she is badly outtimed in the priming battle. This situation calls for desperate measures, as the first rollout below demonstrates. The second and third rollouts are for the other two cube positions.
Blue should double/redouble (and it's a take) whichever 4 White plays. Due to the volatility, Blue's redouble is more correct when White hits, but it is in this way that White salvages the smallest negative equity.
.
1.  Rollout^{1}  Bar/23 6/2*  eq: 0.765 
 Player: Opponent:  30.17% (G:15.79% B:2.72%) 69.83% (G:28.32% B:2.17%)  Conf.: ± 0.009 (0.773...0.756)  [100.0%] Duration: 1 hour 23 minutes 

2.  Rollout^{1}  Bar/23 8/4  eq: 0.812 (0.048) 
 Player: Opponent:  28.62% (G:11.91% B:0.69%) 71.38% (G:18.45% B:0.84%)  Conf.: ± 0.010 (0.822...0.802)  [0.0%] Duration: 1 hour 10 minutes 

3.  Rollout^{1}  Bar/23 9/5  eq: 0.848 (0.083) 
 Player: Opponent:  28.17% (G:11.79% B:0.73%) 71.83% (G:19.42% B:0.89%)  Conf.: ± 0.010 (0.858...0.838)  [0.0%] Duration: 1 hour 07 minutes 

4.  Rollout^{1}  Bar/23 5/1*  eq: 0.870 (0.105) 
 Player: Opponent:  28.58% (G:15.80% B:2.51%) 71.42% (G:30.20% B:2.53%)  Conf.: ± 0.015 (0.885...0.854)  [0.0%] Duration: 1 hour 51 minutes 


^{1} 5184 Games rolled with Variance Reduction. Dice Seed: 85576101 Moves: 3ply, cube decisions: XG Roller

ROLLOUT+C
1.  Rollout^{1}  Bar/23 6/2*  eq: 0.377 
 Player: Opponent:  30.36% (G:15.84% B:2.77%) 69.64% (G:28.09% B:2.20%)  Conf.: ± 0.004 (0.381...0.372)  [100.0%] Duration: 1 hour 21 minutes 

2.  Rollout^{1}  Bar/23 8/4  eq: 0.406 (0.029) 
 Player: Opponent:  28.70% (G:12.03% B:0.70%) 71.30% (G:18.67% B:0.84%)  Conf.: ± 0.005 (0.411...0.401)  [0.0%] Duration: 1 hour 12 minutes 

3.  Rollout^{1}  Bar/23 5/1*  eq: 0.426 (0.049) 
 Player: Opponent:  28.64% (G:16.03% B:2.64%) 71.36% (G:30.23% B:2.37%)  Conf.: ± 0.008 (0.434...0.417)  [0.0%] Duration: 1 hour 49 minutes 

4.  Rollout^{1}  Bar/23 9/5  eq: 0.430 (0.054) 
 Player: Opponent:  28.02% (G:11.60% B:0.73%) 71.98% (G:19.30% B:0.84%)  Conf.: ± 0.005 (0.435...0.425)  [0.0%] Duration: 1 hour 06 minutes 


^{1} 5184 Games rolled with Variance Reduction. Dice Seed: 49207271 Moves: 3ply, cube decisions: XG Roller

1.  Rollout^{1}  Bar/23 6/2*  eq: 0.753 
 Player: Opponent:  30.37% (G:15.84% B:2.77%) 69.63% (G:28.09% B:2.21%)  Conf.: ± 0.009 (0.762...0.744)  [100.0%] Duration: 56 minutes 02 seconds 

2.  Rollout^{1}  Bar/23 8/4  eq: 0.783 (0.030) 
 Player: Opponent:  29.05% (G:12.08% B:0.72%) 70.95% (G:18.38% B:0.75%)  Conf.: ± 0.007 (0.790...0.776)  [0.0%] Duration: 59 minutes 33 seconds 

3.  Rollout^{1}  Bar/23 9/5  eq: 0.860 (0.107) 
 Player: Opponent:  28.02% (G:11.60% B:0.73%) 71.98% (G:19.30% B:0.84%)  Conf.: ± 0.010 (0.870...0.850)  [0.0%] Duration: 50 minutes 57 seconds 

4.  Rollout^{2}  Bar/23 5/1*  eq: 0.878 (0.125) 
 Player: Opponent:  28.43% (G:15.88% B:2.60%) 71.57% (G:30.14% B:2.39%)  Conf.: ± 0.016 (0.895...0.862)  [0.0%] Duration: 38 minutes 59 seconds 


^{1} 5184 Games rolled with Variance Reduction. Dice Seed: 49207271 Moves: 3ply, cube decisions: XG Roller
^{2} 5184 Games rolled with Variance Reduction. Dice Seed: 3541123 Moves: 3ply, cube decisions: XG Roller

The situation above (with White to play 42) somewhat reminds me of the scenario illustrated below that arose 13 years ago:
Entering with the deuce is forced. With the 3, the choice is (a) to button up with 16/13, leaving just one blot, or (b) to hit with 7/4*, leaving five blots. Our choice of (b) was rewarded when our poor opponents rolled 22 (fanning). We won a redoubled gammon (reaching Crawford) and subsequently the match.
Sometimes the best chance to escape a fire is to hurl yourself through it.